1. Field of the Invention
The present invention relates to a table driving device for driving a movable table used in machine tools such as a grinding machine.
2. Description of the Prior Art
Conventionally, mechanisms using a feed screw such as a ball screw are widely employed to move a movable table of the above type. However, where a nut that is threadedly engaged with a feed screw is directly connected to a table, a deflection, torsional moment, vibration, etc. of a nut (hereinafter referred to as deflection components of a nut) associated with rotation of the feed screw are all transmitted to the table, to make it difficult to move the table with high accuracy.
To solve this problem, for example, Japanese Patent Application Unexamined Publication No.63-185550 discloses a movable table which is divided into a main table and a sub-table, and is equipped with a mechanism for coupling these two tables. This coupling mechanism includes a guide joint having a pair of guide surfaces facing each other back to back and perpendicular to the axial line of a feed screw, a presser joint having a pre-load applying means that includes a ball supporting surface opposed to one of the guide surfaces and a ball pressing surface opposed to the other guide surface, a pair of balls interposed between the ball supporting surface of the presser joint and the one guide surface of the guide joint and between the ball pressing surface and the other guide surface of the guide joint. According to this movable table, deflection components of a nut associated with rotation of the feed screw can be absorbed by rolling of the balls.
However, this movable table has a problem that a machine is inevitably made large because the table is divided into the two large parts. Further, since the guide surface for guiding the table and the coupling mechanism for force transmission from the sub-table to the main table are offset from each other, the main table particularly receives such a force as causes pitching (i.e., vibration about the axial line lateral to the table moving direction), so that the accuracy of the table movement is deteriorated.
Further, it is difficult to uniformly apply pre-loads over the entire ball rolling area. Still further, the above coupling mechanism cannot be applied to large-size tables because thrust is given through one point between the pair of balls.